For improving the exhaust emissions of internal-combustion engines, exhaust filters are being used to an increasing extent. These filters primarily hold back soot particles but, with appropriate catalytic coating, can also catalytically burn pollutants existing in the exhaust. These filters have the disadvantage, however, that they clog during the course of time due to the accumulated soot, i.e. the flow resistance for the exhaust increases extremely rapidly. It is therefore attempted to burn the collected soot by effecting a constant or a brief increase of the filter temperature. However, there must be adequate oxygen in the exhaust for burning of the soot to take place. For controlling the oxygen content during burning-off and for producing an optimum fuel mixture, a lambda probe is installed between engine and exhaust filter as an oxygen sensor. The measuring signal of the lambda probe is fed to a control system of the internal-combustion engine which acts in a suitable way on the fresh-air supply and/or the amount of fuel. The customary lambda probes have in their measuring head a ceramic element which is provided with two platinum electrodes and is an ion conductor for oxygen. Examples of ion conductors for oxygen are the ceramic materials ZrO.sub.2, TiO.sub.2, LaF.sub.3, SnO.sub.2, Bi.sub.2 O.sub.3, SrFeO.sub.3, La.sub.x Sr.sub.1-x CrO.sub.3. If there is a different oxygen content in the exhaust and in the surrounding air, the ceramic element in the measuring head causes a diffusion of oxygen ions, which results in a voltage difference between the two platinum electrodes. This voltage difference is available as a measuring signal.
A lambda probe suitable for measuring the oxygen content in the exhaust of internal-combustion engines relative to the oxygen content of the air, with a ZrO.sub.2 ceramic, is known from an article by Hans-Martin Wiedenmann et al., "Heated Zirconia Oxygen Sensor for Stoichiometric and Lean Air-Fuel Ratios", SAE Paper 840141, SAE Congress, Detroit, February-March 1984. The known lambda probe has approximately the shape of a spark plug and can be screwed directly into a bore, provided with an appropriate thread, in a wall of the exhaust system. In the screwed-in state, the ceramic forming the measuring head of the lambda probe protrudes a little into the exhaust chamber. It follows from its functional principle explained above that the measuring signal of the known lambda probe is dependent upon the oxygen partial pressures in the exhaust and in the air. However, the oxygen partial pressure in the exhaust changes with the exhaust pressure. The pressure of the exhaust in the exhaust system of an internal-combustion engine is not constant, but instead depends greatly upon the degree of clogging of the exhaust filter and on the engine speed. In the case of supercharged internal-combustion engines, the pressure variations in the exhaust system are much greater, because the respective charging pressure adds to the influences of the motor speed and the degree of clogging of the exhaust filter. Consequently, the pressure of the exhaust in the exhaust system can vary by a multiple of the air pressure. Under such circumstances the measurement of the percentage oxygen content in the exhaust with a known lambda probe screwed directly into a wall of the exhaust system does not provide usable results. The influence of the exhaust pressure on the measuring signal of the lambda probe could, of course, be eliminated by using a pressure sensor and an electronic calculating unit. such an arrangement however, requires an elaborate design, because the pressure sensor in the exhaust system must be made extremely resistant to corrosion.